This invention relates to position determination apparatus, and to methods of calibrating such apparatus.
Co-ordinate measuring machines (CMM's) and machine tools are two examples of apparatus in which a determination is made of the relative positions of a fixed structure and a movable structure. This may be done either to determine the position of a workpiece-sensing probe provided on one of the structures, or (in the case of a machine tool) to position a cutting tool for a cutting operation. It is known to provide a scale on one of the structures, which has a series of marks. The other structure is provided with a read head for reading the marks on the scale. Normally there are two or three or even more such scale/read head combinations, arranged orthogonally in order to give co-ordinate readings. Various technologies are known for the scale and read head. For example, the scale may comprise optical marks, with an opto-electronic read head. Alternatively, the scale may have magnetic marks, with a magnetic read head.
The accuracy of the apparatus obviously depends on the accuracy of the scale, and careful attention is usually paid to ensuring that the scale is accurately manufactured and calibrated prior to installing it on the machine tool or CMM. However, it is found that inaccuracies can still arise. For example, the scale may become distorted during installation onto machine, or it may be misaligned (e.g. not parallel to the direction of relative motion or not accurately orthogonal to the other scales). Furthermore, inaccuracies or other problems may arise due to misalignment of the read head relative to the scale. Even if accurately installed, inaccuracies can arise during use, for example because of thermal distortion of the scale or the structure to which it is mounted, or distortion due to non-rigidity of the structure under dynamic loads.
It is known (e.g. form U.S. Pat. Nos. 3,555,254 and 3,654,446) to calibrate the machine after the scale has been installed, by using an interferometer to measure those errors noted above which are non-dynamic, at numerous points over the volume of the machine. The first of these patents suggests recording a table of such error value in a computer memory. When the machine is in use, the computer which is controlling the measurement operations can then access the relevent part of the table to obtain an appropriate error value for each scale reading which it takes. The errors in the scale reading can then be corrected automatically by the computer. Unfortunately, a disadvantage of this is the time taken to find and access the appropriate error value in the table, since such error correction needs to operate in real time as the read head is moving over the scale.